‘Listening to the earth’: an unconventional approach to mapping basalt flows

Digitally recorded background ambient noise can be used to extract details regarding subsurface soil conditions. This unique methodology has been implemented to provide comprehensive assessments of geotechnical site conditions. Ambient noise is the persistent vibration of the ground in response to anthropogenic and natural causes. In many contexts, these background vibrations are classified as noise, and efforts are made to remove these signals from recorded data. However, these background vibrations also contain valuable information regarding the materials they travel through. The refraction microtremor (ReMi) technique separates these waves from noise recordings to determine soil shear-wave velocities. Interpolation of closely spaced one-dimensional velocity-depth profiles along linear arrays allow two or three- dimensional velocity-versus-depth representations to be produced, thereby mapping lateral variations and extending subsurface characterisations between more expensive spot borehole measurements. ReMi therefore provides a non- invasive and cost-effective way of characterising ground conditions across entire sites, with data acquisition able to take place in areas where drill-rig access is restricted. This paper demonstrates the contribution ReMi shear-wave velocity assessments can make towards obtaining robust subsurface models to mitigate risk from unforeseen ground conditions in Victoria’s booming construction industry. While traditional seismic refraction methods are unable to detect softer soils underneath harder material, ReMi was able to “see through” the basalt intrusions and delineate the upper and lower surface of the flows. This is particularly advantageous in Victoria, where multiple basalt flows are commonly interlain by soil horizons.